Many engine systems include one or more devices, typically referred to as accessories, which are operated from the engine when the engine is running. Examples of such accessories can include, without limitation, alternators, pumps (including water, air, oil, power steering, brake system and other pump units), air conditioning compressors, superchargers, cooling fans, etc.
Typically these accessories are located adjacent the engine and are directly, or indirectly (via a camshaft, counter balance shaft or idler shaft, etc.) connected to the crankshaft of the engine via a drive means such as a flexible belt, chain or a train of gears such that operation of the engine also drives the accessory through the respective drive means.
While such accessory drive systems have been used for many years, recently fuel efficiency and other concerns have made it desirable to be able to selectively drive some accessories. In other words, it has become desirable to be able to selectively apply engine generated torque to some accessories such that the accessory can be operated, or not, as desired, when the engine is operating. For example, it has become desirable, under some engine and vehicle operating conditions such as cold weather starting, to not operate the water pump for the engine cooling system until the engine has reached a pre-selected operating temperature. Once the engine has reached the pre-selected operating temperature, it is desired to apply torque from the engine to the water pump to operate the pump.